1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to an organic electroluminescent display device and method of fabricating an organic electroluminescent display device.
2. Discussion of the Related Art
Flat panel display devices, which are characterized as having a thin profile, light weight, and energy efficient, can be classified into one of two types depending on whether it emits or receives light. A first type is a light-emitting type that emits light to display images, and a second type is a light-receiving type that uses an external light source to display images. Plasma display panels, field emission display devices, and electroluminescence display devices are examples of the light-emitting type display devices, whereas liquid crystal displays are examples of the light-receiving type display devices.
Among the different types of flat panel display devices, liquid crystal display (LCD) devices are commonly used for laptop computers and desktop monitors because of their high image resolution, good color production, and superior image quality. However, the LCD devices have some disadvantages, such as poor contrast ratios, narrow viewing angles, and limited sizes. Accordingly, new types of flat panel displays are required to overcome these disadvantages, but yet still maintain a thin profile, light weight and have low power consumption.
Organic electroluminescent display (OELD) devices have been developed because of their wide viewing angles and good contrast ratios, as compared to the LCD devices. The OELD devices are light-emitting type display devices that do not require a backlight device, and are light weight and have a thin profile. In addition, the OELD devices have low power consumption, wherein a low voltage direct current can be used to drive the OELD devices while obtaining rapid response speeds. Since the OELD devices are solid state devices, unlike the LCD devices, they are sufficiently strong to withstand external impact, have relatively larger operational temperature ranges, and can be manufactured at lower costs than LCD devices. Moreover, since only deposition and encapsulation apparatus are necessary and injection of liquid crystal materials is unnecessary, process management is simpler than the manufacturing of LCD devices.
One method for operating an OELD device is a passive matrix operating method that does not utilize thin film transistors, wherein scanning lines and signal lines are arranged in a matrix configuration to perpendicularly cross each other and a scanning voltage is sequentially supplied to the scanning lines to operate each pixel. In order to obtain a required average luminance, the instantaneous luminances of each pixel during a selected period is intensified by increasing the number of scans during the period.
Another method of operating an OELD device is an active matrix operating method, wherein thin film transistor pairs, which create a voltage storing capability for each of the pixels, include a selection transistor and a drive transistor. The source/drain of the selection transistor is connected to a signal line for supplying a data signal when a scanning signal is supplied to the gate scanning line, and the gate of the drive transistor is connected to the source/drain of the selection transistor and a constant voltage line is connected to the source/drain of the drive transistor. In the active matrix type OELD device, a voltage supplied to the pixels is stored in storage capacitors, thereby maintaining the signals until a next period for supplying a signal voltage begins. As a result, a substantially constant current flows through the pixels, and the OELD device emits light at a substantially constant luminance during one frame period. Since a very low current is supplied to each pixel of an active matrix type OELD device, it is possible to enlarge the display device to form finer and/or larger image patterns having low power consumption.
FIG. 1 is a schematic circuit diagram of a pixel region in an active matrix type OELD device according to the related art. In FIG. 1, scanning lines are arranged along a transverse direction, and signal lines are arranged along a longitudinal direction perpendicular to the scanning lines. A power supply line is connected to a power supply to provide a voltage to drive transistors and is disposed along the longitudinal direction, wherein a pixel region is defined between a pair of signal lines and a pair of scanning lines. Each selection transistor, which is commonly known as a switching thin film transistor (TFT), is disposed in the pixel region near the crossing of the scanning line and the signal line and functions as an addressing element to control the voltage of a pixel electrode. A storage capacitor CST is connected to the power supply line and the drain/source of the switching TFT. Accordingly, each drive transistor, which is commonly known as a driving TFT, has a gate electrode connected to the storage capacitor CST and a source/drain connected to the power supply line and functions as a current source element for the pixel electrode.
In FIG. 1, an organic electroluminescent diode is connected to the drain/source of drive transistor, and includes a multi-layer structure having organic thin films provided between an anode electrode and a cathode electrode. When a forward current is supplied to the organic electroluminescent diode, electron-hole pairs combine in an organic electroluminescent layer as a result of formation of a P-N junction between the anode electrode, which provides holes, and the cathode electrode, which provides electrons. The electron-hole pairs have a larger energy together when combined than when they were separated. The energy gap between combined and separated electron-hole pairs is converted into light by an organic electroluminescent element. That is, the organic electroluminescent layer emits the energy generated due to the recombination of electrons and holes when a current flows.
Organic electroluminescent devices are classified into top emission type and bottom emission type in accordance with a progressive direction of light emitted from the organic electroluminescent diode. In the bottom emission type device, light is emitted along a direction toward the substrate where the various lines and TFTs are disposed. However, in the top emission type device, light is emitted along a direction opposite to the substrate where the lines and TFTs are disposed.
FIG. 2 is a partial cross sectional view of a bottom emission type OELD device including one pixel region having red (R), green (G), and blue (B) sub-pixel regions according to the related art. In FIG. 2, first and second substrates 10 and 30 are bonded to each other using a seal pattern 40, wherein thin film transistors T and first electrodes 12 are formed on the first substrate 10, which is transparent. The pixel of the organic electroluminescent device generally includes three sub-pixel regions with the thin film transistor T and one of the first electrodes 12 disposed within each sub-pixel region. An organic electroluminescent layer 14 is formed over the thin film transistors T and over the first electrodes 12 and includes luminous materials that produce red (R), green (G), and blue (B) colors each corresponding to each thin film transistor T within each sub-pixel region. A second electrode 16 is formed on the organic electroluminescent layer 14, wherein the first and second electrodes 12 and 16 supply the electric charges to the organic electroluminescent layer 14.
The seal pattern 40 bonds the first and second substrates 10 and 30 together to maintain a cell gap therebetween. Furthermore, a hygroscopic material or a moisture absorbent material 22 is formed on an inner surface of the second substrate 30 in order to absorb any moisture within the cell gap between the first and second substrates 10 and 30 to protect the cell gap from moisture. In addition, a translucent tape 25 is formed on the second substrate 30 and the hygroscopic material 22 is tightly adhered to the second substrate 30.
In FIG. 2, if the first electrode 12 is an anode and the second electrode 16 is a cathode, the first electrode 12 is formed of a transparent conductive material and the second electrode 16 is formed of a metal having a small work function. Furthermore, the organic electroluminescent layer 14 includes a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d in a sequential order from the first electrode 12. Accordingly, the emission layer 14c includes luminous materials that emit red (R), green (G), and blue (B) colors in an alternate order in the corresponding sub-pixel regions.
FIG. 3 is an enlarged cross sectional view of one pixel of the bottom emission type OELD device of FIG. 2 according to the related art. In FIG. 3, an organic electroluminescent display device includes a thin film transistor (TFT) T and an organic electroluminescent diode E in a luminous emitting area L. In addition, a buffer layer 32 is formed on the transparent substrate 10, wherein the TFT T includes a semiconductor layer 62 on the buffer layer 32, a gate electrode 68, a source electrode 80, and a drain electrode 82. A power electrode 72 extends from the power supply line to be connected to the source electrode 80, and the organic electroluminescent diode E is connected to the drain electrode 82. A capacitor electrode 64 made of the same material as the semiconductor layer 62 is disposed below the power electrode 72. The power electrode 72 corresponds to the capacitor electrode 64, and an insulator is interposed therebetween, thereby forming a storage capacitor CST.
The organic electroluminescent diode E includes the first electrode 12, the second electrode 16, and the organic electroluminescent layer 14 interposed between the first electrode 12 and the second electrode 16. The OELD device shown in FIG. 3 includes the luminous area L where the organic electroluminescent diode E emits light produced therein. Furthermore, the organic electroluminescent display device has array elements A that include the TFT T, the storage capacitor CST, and various lines and insulators upon which the organic electroluminescent diode E is disposed. Accordingly, the organic electroluminescent diode E and the array elements A are formed on the same substrate.
FIG. 4 is a flow chart showing a fabrication process of the OELD device of FIG. 3 according to the related art. In FIG. 4, step st1 includes a process for forming the array elements on a first transparent substrate. For example, the scanning lines, the signal lines, and the switching and driving thin film transistors are formed on and over the first substrate, wherein the signal lines are formed perpendicularly to cross the scanning lines. Each of the switching thin film transistors is disposed near a crossing of the scanning and signal lines. In addition, formation of the array elements also includes forming the storage capacitors and the power supply lines, wherein each of the driving thin film transistors is disposed near a crossing of the scanning and power supply lines.
A step st2, includes formation of the first electrodes of the organic electroluminescent diode, wherein the first electrode is provided within each of sub-pixel regions. Each of the first electrodes are also connected to the drain/source of the driving thin film transistor within each of the sub-pixel regions.
A step st3 includes formation of an organic electroluminescent layer on the first electrodes. If the first electrodes are the anode, then the organic electroluminescent layer is formed to have a sequential multiple structure of a hole injection layer, a hole transporting layer, an emission layer, and an electron transporting layer on the first electrode. Conversely, if the first electrodes are the cathode, the sequence is reversed.
A step st4 includes forming the second electrode of the organic electroluminescent diode on the organic electroluminescent layer to cover an entire surface of the first substrate, wherein the second electrode functions as a common electrode.
A step st5 includes encapsulation of the first and second substrates, wherein a second substrate is bonded to the first substrate having the array elements and the organic electroluminescent diode. The second substrate protects the organic electroluminescent diode of the first substrate from external impact. Accordingly, since the first substrate is encapsulated with the second substrate, the organic electroluminescent diode is protected from an ambient atmosphere exterior to the device. As previously described, the second substrate includes the hygroscopic material on the inner surface thereof.
A yield of an OELD device is determined by both the yield of array elements and the yield of the organic electroluminescent layer, wherein the fabrication yield of the organic electroluminescent layer determines and controls the total fabrication yield of the OELD device. For example, although the thin film transistors are formed without any defects on the first substrate, the first substrate having both the array elements and the organic electroluminescent layer can be determined to be inferior if defects are generated during later processes for forming the organic electroluminescent layer. Thus, it is a waste of time and cost to fabricate the array elements on the first substrate when defects later occur in the organic electroluminescent layer during the fabrication.
Moreover, in the bottom emission type device, light is emitted along the direction toward the substrate where the lines and TFTs are disposed. Therefore, the display area decreases because the emitted light is blocked by these lines and TFTs. In the top emission type device, since light is emitted along the direction opposite to the substrate where the lines and TFTs are disposed, the display area can increase and thereby simplifying design the TFTs. However, since the top emission type OELD device has the cathode electrode on the organic electroluminescent layer, the cathode electrode is commonly formed of a transparent or translucent material that may block some of the light emitted from the organic electroluminescent layer that decreases light efficiency. Furthermore, there are some limitations in selecting the transparent or translucent material for the cathode second electrode.
To prevent a reduction of light permeability, a thin film passivation layer may be formed over an entire surface of the substrate. However, infiltration of ambient atmosphere from an exterior of the device is not prevented and affects the organic electroluminescent diode.